1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method that process image data corresponding to unit areas to form an image in the unit areas by moving a print head a plurality of times relative to the unit areas on a print medium or by moving a plurality of print heads relative to the unit areas.
2. Description of the Related Art
As one printing method using a print head having a plurality of printing elements, an ink jet printing method that ejects ink from the individual printing elements to form dots on a print medium is known. Such ink jet printing apparatus can be classified into a full-line type and a serial type in terms of their construction.
A full-line type printing apparatus uses a print head having a plurality of printing elements arranged over as wide a width of the print medium. Then, the print head ejects ink as the print medium is moved in a direction crossing the direction of arrangement of the printing elements in the print head to form an image on the print medium. Such a full-line type printing apparatus can produce an image at a relatively high speed and therefore is suited for office use.
In a serial type printing apparatus, on the other hand, an image is formed progressively by repetitively performing a printing scan for scanning (moving) the ink ejecting print head and a print medium conveying operation for conveying the print medium in a direction crossing the printing scan. Such a serial type printing apparatus can be manufactured in a relatively small size and at a low cost and therefore is suited for a personal use and for use as a wide format printer.
Whether it is of the full-line type or a serial type, there are variations in ink ejection volume and direction among the individual printing elements arrayed in the print head. Such variations may cause density variations or stripes on a printed image.
As a technology to alleviate such image problems, a multipass printing method has been known. In the multipass printing, image data to be printed in a unit area of the print medium is divided into as many pieces of image data as a plurality of relative scans and the divided pieces of image data are sequentially printed by the plurality of relative scans with a print medium conveying operation interposed in between, thus completing the image to be printed in the unit area. Such a multipass printing method can alleviate image problems caused by ejection variations among different printing elements. As a result, a uniform, smooth image can be obtained. The multipass printing produces greater effects as the number of passes, i.e., the number of printing elements used to print one scan raster, is increased. It is noted, however, that since the increased number of passes results in a reduced printing speed, the serial type printing apparatus often provides multiple print modes with different number of passes, such as one giving priority to the image quality and one giving priority to the printing speed.
The above multipass printing method can be applied to the full-line type printing apparatus. That is, as shown in FIG. 1, two or more printing element arrays of each color are arranged in a print medium conveying direction so that a rasterized line can be printed by a plurality of printing elements, alleviating adverse effects caused by ejection variations among individual printing elements.
In performing the multipass printing described above, it is necessary to distribute image data among individual printing scans in the serial type printing apparatus and, in the full-line type printing apparatus, to distribute image data among individual print heads. Such a distribution of image data has often been performed by using a mask pattern comprising printable pixels (1) in which a dot is permitted to be printed and unprintable pixels (0) in which a dot is not permitted to be printed.
FIG. 13 shows one example of the mask pattern that can be used in a 2-pass printing. Areas painted black represent the printable pixels (1) and blank areas represent the unprintable pixels (0). Denoted 1801 is a mask pattern used for a first pass of the printing scan and 1802 for a second pass. The pattern 1801 and the pattern 1802 are in a complementary relationship.
By performing an AND operation between the mask patterns and binary image data, the binary image data is divided into pieces that are to be printed by different printing scans. For example, as shown in FIG. 2, the image data representing dots to be printed in a unit area is divided by mask patterns (1801, 1802) of FIG. 13 to generate divided image data for 1st pass and divided image data for 2nd pass. In this mask-based data dividing method that uses complementary mask patterns, the possibility of dots printed in different scans overlapping each other is low since the binary image data assigned to different scans also have a complementary relationship.
With demands for even higher image quality growing in recent years while the multipass printing is employed, density variations or density uneveness caused by registration errors (print position deviations) among different printing scans or among different printing element arrays have come to be spotlighted as problems. The print position deviations among different printing scans or among different printing element arrays are caused by variations in distance between a print medium and an ejection opening face of the print head (head-medium distance) and by variations in the distance that the print medium is conveyed.
For example, referring to FIG. 2, let us consider a case where a dot (◯) plane printed in a preceding printing scan and a dot (⊙) plane printed in a subsequent printing scan are shifted by one pixel either in the main scan direction or subscan direction. At this time, the dots (◯) printed in the preceding printing scan and the dots (⊙) printed in the subsequent printing scan completely overlap, leaving blank areas exposed, lowering the density or grayscale value of a printed image. Even if the two planes of dots are not shifted by as large as one pixel, variations in adjoining dot distance and in overlapping amount change a dot coverage over the blank areas, which in turn causes variations in grayscale value of the image. Such grayscale value variations appear as density uneveness.
Therefore, with higher quality of images being called for in recent years, there is a growing demand for an image data processing method in a multipass printing that can deal with print position deviations between different dot planes caused by variations of printing conditions. In the descriptions that follow, a capability to oppose density variations or density unevenness caused by inter-plane print position deviations, whatever printing condition variations they may be caused by, is referred to as a “robustness”.
Japanese Patent Laid-Open No. 2000-103088 discloses an image data processing method for enhancing the robustness. This patent document focuses on the fact that image density variations caused by variations in print condition stem from different pieces of binary image data used in different printing scans being in a complementary relationship. The patent document recognizes that generating pieces of image data used in different printing scans in ways that will reduce the complementary relationship can realize a highly robust multipass printing. Japanese Patent Laid-Open No. 2000-103088 therefore divides the image data in the form of multivalued data before binarization and then independently binarizes the divided pieces of multivalued data, thereby preventing large density variations from occurring even if different planes of image data used in different printing scans are printed deviated from each other.
FIG. 3 shows the process of data division described in Japanese Patent Laid-Open No. 2000-103088. First, the multivalued data to be printed in a unit area (see A) is divided into a piece of multivalued data to be printed in a first pass (see D) and a piece of multivalued data to be printed in a second pass (see E). Next, these divided multivalued data are individually binarized to create a piece of binary data to be printed in the first pass (see H) and a piece of binary data to be printed in the second pass (see I). Finally, the print head ejects ink according to these binary data. As can be seen from (H) and (I), the first pass binary data and the second pass binary data created as described above are not in the complementary relationship. Therefore, locations where dots of the first pass and the second pass overlap (i.e., pixels that have “1” in both of the two planes) and locations where dots of the first and second pass do not overlap (i.e., pixels that have “1” in only one of the two planes) exit simultaneously.
FIG. 4 shows dots printed on a print medium according to the method of Japanese Patent Laid-Open No. 2000-103088. In the figure, black circular dots 21 represent dots printed in the first pass, blank circular dots 22 represent dots printed in the second pass and hatched circular dots 23 represent dots printed overlappingly in the first and second pass. In this example, since the complementary relationship between the first pass dots and the second pass dots is incomplete, as opposed to the case of FIG. 2 where the first and the second pass dots are in a complete complementary relationship, there are areas where two dots overlap and areas where no dots are printed (blank areas).
Here, let us consider a case in which dots printed in the first pass and dots printed in the second pass are shifted by one pixel either in the main scan direction or in the subscan direction, as in the case of FIG. 2. In that case, the first and second pass dots that are supposed not to overlap if the print position deviation does not occur, now overlap. At the same time, other dots—the dots 23 that are supposed to overlap if the print position deviation does not occur—do not overlap. Therefore, considering a certain expanse of printed area, the dot coverage over the blank areas changes little and therefore the image density change is small. That is, with the method of Japanese Patent Laid-Open No. 2000-103088, if the distance between the print medium and the ejection opening face (head-medium distance) changes or if the print medium conveying distance changes, it is possible to prevent image density variations that may be induced by these changes.
Further, Japanese Patent Laid-Open No. 2006-231736 discloses the technology that distributes pieces of image data in the form of multivalued data to a plurality of printing scans or a plurality of printing element arrays, like Japanese Patent Laid-Open No. 2000-103088, and at the same time changes a distribution ratio of the multivalued image data according to the position of pixels. This patent document describes its capability to limit banding and color banding in the multipass printing by changing the distribution ratio according to the pixel positions in the main scan direction, linearly, cyclically, sinusoidally or based on a combination of high and low frequency waves.
However, even with Japanese Patent Laid-Open No. 2000-103088 and Japanese Patent Laid-Open No. 2006-231736 (multivalued data dividing method), the inventors of this invention have found that, when a grayscale value of the image data is low (image density is low), image impairments may emerge in an output image. The image impairments are described as follows.
FIG. 5A shows how an image is processed and how dots are printed when the image data is divided in the form of multivalued data into two planes before being binarized by an error diffusion method. FIG. 5B shows how an image is processed and how dots are printed when the image data is directly binarized without being divided into two planes. Here, an original image 50 is assumed to be a uniform half-tone image having a relatively low grayscale value of 11/255 in a grayscale range of 0-255. According to the method of Japanese Patent Laid-Open No. 2000-103088, the original image 50 in the multivalued state is divided into two planes 51a and 51b having a grayscale value of 5/255 and 6/255 respectively. Then, the two planes are subjected to the binarization operation based on the error diffusion method to create binary plane images 52a and 52b. These two plane images 52a and 52b are printed overlappingly to produce an output image 53.
FIG. 5B shows image processing and a printed state when the image data is directly binarized based on the error diffusion method without being subjected to the plane division. The original image 50 at the grayscale value of 11/255 is binarized, without being divided into planes, to generate an output image 54.
Here, comparison between the output images 53 and 54 shows that there are more blank areas in an upper end portion of the output image 53 than in the output image 54. This is due to the fact that dots are not arranged in the upper end portion of any of the two plane images 52a and 52b before being overlapped. Such a dot arrangement is characteristic of the image processing that employs the error diffusion method.
Where the binarization is based on the error diffusion method, whether a dot is to be printed or not in individual pixels is determined by whether the grayscale value of each pixel of interest is higher than a predetermined threshold (e.g., 128). More specifically, when the grayscale value is higher than the threshold, it is determined that a dot shall be printed (255). When it is lower than the threshold, it is determined that a dot shall not be printed (0). An error between the output value and the input value in each pixel of interest is distributed to surrounding pixels that are not yet binarized, so that a certain range of grayscale value is stored before and after the binarization operation. At this time, when an image has relatively low, uniform grayscale values, the binarization operation is performed for many pixels until the error is accumulated to exceed the threshold. Generally, the pixel of interest moves from left to right and from top to bottom, so the position at which a first dot is printed is some distance from the top edge of the print medium, as shown in FIG. 5A. Therefore, the lower the grayscale value (or density) of each pixel, the smaller the error that is accumulated and the larger the distance from the top edge of the print medium. Here, a phenomenon in which the pixel printing is delayed as described above is referred to as a “dot generation delay”.
That is, the division of the original image 50 with a grayscale value of 11 into two planes, as shown in FIG. 5A, has contributed to the dot generation delay, increasing the blank portion near the top edge of the print medium. As can be seen from the comparison between the output image 53 and the output image 54, such a dot generation delay hinders an appropriate dispersion of dots, resulting in a loss of uniformity of the image. This phenomenon is similarly observed with multipass printing of more than two passes.
On the other hand, with the construction in which there is no correlation of binary data among a plurality of planes, such as shown in Japanese Patent Laid-Open No. 2000-103088, a graininess of the printed image may get worse in areas where the grayscale values are low. For example, referring again to FIG. 5A and FIG. 5B, since the binary plane images 52a and 52b are not correlated and are independently binarized, there are areas in the combined output image 53 where a plurality of dots overlap or lie side by side. Therefore, compared with the output image 54 in which all dots are uniformly dispersed with the errors of all dots diffused, the combined output image 53 shows dot aggregates, degrading the graininess.
Such degraded graininess is caused not by the binarization method but by the fact that a plurality of planes have no correlation, such as a complementary relationship. So, if a binarization method other than the one based on the error diffusion method shown in FIGS. 5A and 5B is employed, the above image impairment still emerges. For example, in a dither matrix method, when relatively similar matrices are used, a probability of dots on different planes overlapping each other increases, further deteriorating the graininess.
The multivalued data dividing method of Japanese Patent Laid-Open No. 2000-103088 is most likely to be effective when the grayscale value is such that the dot overlapping state greatly affects the dot coverage over the blank areas, e.g., when the grayscale value results in the dot coverage of about 30% to 60%. However, in an image with low grayscale values, such as shown in FIGS. 5A and 5B, the distances between dots are relatively large so that shifts between different planes do not result in a sharp density variations. In this case, therefore, the method is not likely to show its effectiveness. Further, in low-grayscale images, since dots are sparsely scattered and the dot-to-dot distances are large, even if dots arrayed in the main scan direction are printed by the same printing element, image impairments such as stripes are hardly recognized. This means that the multipass printing is not likely to show its effectiveness in such low-grayscale images.
On the other hand, in low-grayscale images, dot dispersion and graininess often become sensitive issues attracting attention and can be degraded by the dot generation delay shown in FIG. 5A and aggregates of multiple dots.